1. Technical Field
The present invention relates to a method of making absorbent articles which contain thermoplastic materials. More specifically, the invention relates to a method of making single or multiple layer composite articles which include at least one layer formed from a mixture of thermoplastic and other nonthermoplastic fibers, such as wood pulp fibers.
2. Description Of The Prior Art
Although methods of forming articles and materials of a combination of thermoplastic and other fibers, such as wood pulp fibers, are known, these prior articles and materials produced by such methods suffer from a number of disadvantages.
U.S. Pat. No. 4,458,042 of Espy discloses an absorbent material comprised of a consolidated blend consisting essentially of wood pulp fluff and wetting agent treated spurted polyolefin pulp. The polyolefin pulp is from about 3% to about 30% of the total weight of the blend. Representative polymers for the spurted polyolefin pulps include polyethylene, polypropylene and copolymers of ethylene and propylene. Mixtures of two or more of these polymers are also described as a suitable polyolefin pulp. The polyolefin pulp and wood pulp are blended, formed into a fluff pad and then consolidated by heating to a temperature above the melting point of the polyolefin pulp. Calenders, infrared heaters and pull-through dryers are described as representative heating devices.
Although useful, absorbent materials formed in this manner tend to lose fibers from their outer edges, particularly when shaken. In addition, pads of consolidated materials formed in this manner have a relatively low Z direction tensile strength which makes them relatively easy to pull apart, especially at the peripheral edges. Also, the method of producing pads in this manner results in pads which do not impede the leakage of liquid deposited on these materials from their outer edges.
U.S. Pat. No. 4,609,580 of Rockett et al. discloses an absorbent floor mat comprising a combination of a nonwoven liquid permeable wear surface, an absorbent inner layer of a mixture of polymeric microfibers and wood pulp, and a liquid impervious film backing layer. Intermittent bonds within the periphery or field of the floor mat are provided. These bonds are formed by a patterned application of sonic energy or heat and pressure.
In Rockett et al., a nonwoven web of nylon, such as sold under the trademark Cerex.RTM. from James River Corporation is listed as one example of a liquid permeable layer. The absorbent microfiber layer is described by Rockett et al. as being an essential feature of this mat. This layer is described preferably as an admixture of thermoplastic microfibers and "other" fibers such as wood pulp or natural or synthetic staple fibers. The absorbent layer is stated to have a basis weight in the range of from about 100-500 g/m.sup.2 and preferably in the range of about 150-250 g/m.sup.2. The composition of this layer is described as ranging from about 0-80% of the "other" fibers and preferably in the range of from about 60-80% wood pulp fibers by weight. The microfibers are described as preferably being of thermoplastic polymers such as polyolefins, polyesters or polyamides having a diameter on the average in the range of up to about 15 microns and preferably in the range of up to about 10 microns. Polyethylene and polypropylene microfibers are identified as specific examples.
The absorbent layer is described in this patent as preferably being formed in accordance with the "coform" process described in U.S. Pat. No. 4,100,324 of Anderson et al. In the Anderson conform approach, streams of molten polymer are deposited in an airstream and combined by a secondary air stream containing, for example, wood pulp fibers. A combination of the air streams causes the distribution of the wood pulp in the microfiber matrix. In addition, exemplary staple fibers, if included in the "other" fibers, are listed as polyester, polyolefins, polyamides and mixtures thereof. Finally, the liquid impermeable surface is described as preferably being a film with examples being thermoplastic polymers such as polyolefins, polyesters and the like, including polyethylene or polypropylene films. The film is described as being applied as a separate layer, coextruded, or coated onto the absorbent web. Calendering the exposed absorbent surface or providing a bottom adhesive layer are described as alternate ways of achieving liquid imperviousness of the underside of the floor mat.
The Rockett et al. floor mat has field bonds occupying up to about 10-25% of the surface of the floor mat. If an opened, disconnected pattern of field bonds is used, Rockett et al. discloses that up to about 20 bonds per square inch are provided. If a line pattern is used as described in Rockett et al., the pattern is up to about 10 lines per inch on the average in any direction. In use, the floor mat may be positioned in a holder which surrounds the peripheral edge of the mat.
In forming an absorbent layer in the manner of U.S. Pat. No. 4,100,324 of Anderson et al, the meltblown microfibers are softened, but are not above their melting point when they are engaged by wood pulp or the "other" fibers. Consequently, the bonding that occurs between these microfibers and the "other" fibers is relatively weak in comparison to the bonding that results when a thermobonding approach is used. Thermobonding in this sense means raising the temperature of a mixture of thermoplastic and other fibers to a temperature which is above a melt point of at least one of the thermoplastic fibers in the mixture. When this happens, a much stronger fusing of the mixture results. In addition, by relying on field bonds to secure the floor mat together, the Z direction tensile strength of the Rockett et al. composite mat is relatively weak. Moreover, the peripheral edges of the Rockett et al. floor mat are as weak as the interior areas of the mat and would not impede the leakage of liquid from these edges.
Another example of a pad which exemplifies the prior art is described in U.S. Pat. No. 4,650,481 of O'Connor et al. The pad of O'Connor et al has a liquid impermeable backing sheet, an overlaying liquid permeable face sheet and an absorbent conform layer between the backing and face sheets The interior of the pad is provided with a quilted pattern of compression lines described as being formed by ultrasonic bonding, heat and compression or the use of glue and compression. In an illustrated example, the pad is generally rectangular and the quilting lines appear to form a pattern of squares on the pad. The backing sheet is described as being generally bonded to the absorbent material by adhesive.
The coform absorbent material of the O'Connor et al. patent is described as being of meltable polymers and staple fibers formed as disclosed in U.S. Pat. No. 4,100,324 of Anderson et al. Typical polymers are described as polyethylene, polyesters, nylon and other thermoplastic fibers. Staple fibers are described as including cotton, polyester, rayon, and nylon. A combination of polypropylene meltblown fibers and wood pulp fibers is described as preferred in any desired ratio, but preferably with meltblown polypropylene fibers being present in an amount from between about 30% and about 40% by weight of the mixture. Examples of the backing sheet in O'Connor et al. include polymer films, such as copolymers of ethylene and vinyl acetate, nylon and polyesters. The preferred backing sheet films are identified in this patent as being of polyethylene or polypropylene and a composite of polypropylene and a lightweight spun bonded fabric. Spunbonded polypropylene is listed in this patent as one example of a facing sheet.
During one method of manufacturing the O'Connor, et al. pad, the facing sheet is placed on a foraminous belt with meltblown polypropylene and wood fibers being deposited onto the facing sheet as it moves below meltblown producing nozzles. The coform thus becomes mechanically attached to the facing sheet. The combined coform and facing sheet is brought in contact with an adhesively coated polymer backing sheet which is secured to the coform side of the combination. The pad is then embossed to form the quilting pattern.
The O'Connor et al. pad produced in this manner suffers from many of the drawbacks of the Rockett et al. floor mat discussed above. For example, coform provides relatively weak bonding of a pad. In addition, there is a tendency of the pad of O'Connor et al. to leak at the edges. O'Connor, et al. recognizes this and describes an embodiment (FIGS. 6 and 7) directed toward solving this problem. In this embodiment, the absorbent material is centered but does not extend completely to the peripheral edge of the pad. Instead, the facing and backing sheets are directly connected at the edge of the pad.
Therefore, although thermoplastic fibers have been combined with other fibers and used in the manufacture of articles, a need exists for improved methods for making such materials and articles.